The cutting edges of one type of standard drill are produced by the intersection of two or more axially extending flutes with the conical head of the cylindrical body of the drill. The straight cutting edges so created run from the apex of the conical head to the cylindrical side wall of the drill body, creating an obtuse angled corner. In some cutting applications, it is desired to give the cutting edges more wear resistance than the material of the drill body alone could provide. Since it is the outboard portion of the cutting edges that moves at the highest cutting speed, and therefore sees the most potential wear, it is often enough to enhance just that outboard portion, creating what may be referred to as an inserted drill. This is typically done by machining a pocket with an arcuate back edge into the face of the flute, across the corner. An insert of more wear resistant material is cut to basically the same size as the pocket, with an arcuate back edge of substantially the same radius and curvature. When attached into the pocket, an edge of the insert will be aligned with what remains of the original cutting edge, becoming part of it, in effect.
The insert is typically attached to the drill body by a brazing process. In the brazing process, a piece of brazing material cut from a thin sheet to approximately the same shape as the pocket is placed in the pocket and liquefied by melting it with a torch. Then, the insert is delicately laid into the pocket on top of the melted layer of brazing material by an operator working with a tweezer like tool. The insert in effect floats on the melted layer as it is carefully manipulated until its outer straight edge is aligned with the cutting edge of the drill body. As a practical matter, it is not possible to machine the pocket and insert arcuate edges closely enough that they can be pushed hard against one another with no gap therebetween. As a result, the operator is obliged to hold a semi annular gap between the two arcuate edges as the insert is maneuvered. it is difficult to minimize that gap, since there is no positive stop to work against, and because the operator has to worry about aligning of the rest on the insert, as well. consequently, brazing material inevitably is squeezed into and hardens in the gap between the arcuate edges, creating a semi annular seam, one end of which intersects the cutting edge. The brazing material is much softer than either the drill material or the insert, and is therefore subject to wear and erosion from the workpiece as the drill cuts.